Description: Special subclass of PropagationEffectParameter.
This computes the closest distance to the seimogenic part of the fault;
that is, the closest distance to the part of the fault that is below the seimogenic
thickness (seisDepth); this depth is currently hardwired at 3 km, but we can add
setSeisDepth() and getSeisDepth() methods if desired (the setter will have to create
a new constraint with seisDepth as the lower bound, which can be done even if the
parameter has been set as non editable). Note that if the earthquake rupture is a
line or point source where the depths are less than seisDepth, then the depths are
treated as seisDepth (e.g., for grid based forecast where all sources are put at
zero depth)

clone

Note: this is not a true clone. I did not clone Site or ProbEqkRupture.
PE could potentially have a million points, way to expensive to clone. Should
not be a problem though because once the PE and Site are set, they can not
be modified by this class. The clone has null Site and PE parameters.

This will probably have to be changed in the future once the use of a clone is
needed and we see the best way to implement this.